1. Field of the Invention
The present invention relates to ion-exchange membranes for electrochemical fuel cells. More particularly, the invention improves electrochemical fuel cell performance by impregnation of the ion-exchange membrane.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes namely a cathode and an anode. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. In addition to electrocatalyst, the electrodes may also comprise an electrically conductive substrate upon which the electrocatalyst is deposited. The electrocatalyst may be a metal black (namely, a substantively pure, unsupported, finely divided metal or metal powder) an alloy or a supported metal catalyst, for example, platinum on carbon particles.
A solid polymer fuel cell is a type of electrochemical fuel cell which employs a membrane electrode assembly (“MEA”). The MEA comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. The ion-exchange membranes of particular interest are those prepared from fluoropolymers and which contain pendant sulfonic acid functional groups and/or carboxylic acid functional groups. A typical perfluorosulfonic acid/PTFE copolymer membrane can be obtained from DuPont Inc under the trade designation Nafion®.
A broad range of reactants can be used in electrochemical fuel cell. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute oxygen stream such as air.
The electrochemical oxidation which occurs at the anode electrocatalyst of a solid polymer electrochemical fuel cell, results in the generation of cationic species, typically protons. The cations must then cross the membrane to the cathode electrocatalyst where reaction with the oxidant generates water thereby completing the electrochemistry. Typically, transport of cations across the membrane is assisted by water molecules in the membrane. Humidification of the membrane thus improves fuel cell performance.
One method of increasing the electrical conductivity of the membrane is disclosed in U.S. Pat. No. 3,684,747 in which a swelling agent is used to increase the liquid absorptive capacity of the polymer. An increase in the absorption of aqueous electrolyte by the polymer increases the electrical conductivity of the polymer.
Unfortunately, conductivity suffers at higher temperatures, particularly over 100° C. where there is reduced water absorption. As the vapor pressure of water increases rapidly with temperature, it becomes much more difficult to operate at higher temperatures. There is also a general desire to operate under low relative humidity conditions even at normal operating temperatures. Various approaches have been undertaken to improve fuel cell performance under high temperature—low humidity conditions such as, for example, phosphoric acid doped membranes. However, acid doped membranes tend to have a high degradation rate with corrosion of cell components. High temperature operation has been observed with membranes swollen with ionic liquids, such as 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate (BMITf) and 1-butyl, 3-methyl imidazolium tetrafluoroborate (BMIBF4). However, BMITf and BMIBF4 are highly toxic compounds that may leach out of the fuel cell during operation.
Generally, amines have been considered to be either non-conductive or have only a low proton conductivity. An exception noted by K. D. Kreuer et al., Electrochimica Acta 43(10-11):1281, 1998, involves imidazole and pyrazole in which relatively high proton conductivity has been observed in sulfonated polyetherketo-membrane systems. Both imidazole and pyrazole are heterocycles with the following structures: K. D. Kreuer et al. attributed the high conductivity to imidazole and pyrazole each having a non-polar ring and both protonated and unprotonated nitrogen functionality. Thus imidazole and pyrazole may act as both hydrogen donors and acceptors in proton conduction processes. While these compounds may show increased conductivity within membrane systems, it is unlikely that they are suitable for use within the fuel cell environment. For example, a recent study by C. Yang et al., Journal of Power Sources 103:1, 2001, reports that imidazole impregnated membranes poisoned the catalysts.
There continues to be a need for membrane additives that improve electrochemical performance and are suitable for use within the fuel cell environment.